refactor(ui): extract PopupMenu::beginConfirm() for destructive-action popups

Found while re-reviewing this session's own commits: five screens each
hand-built the same 2-row Action/Cancel confirm popup, defaulting the
highlight to Cancel -- NearbyScreen's contact-delete, AdminScreen's
OTA-start, and the three just added (Trail's reset, Messages' channel
delete, RadioPresetPicker's preset delete). The plan that added those
three had already flagged this exact duplication without acting on it, so
it just tripled instead of getting fixed.

One PopupMenu::beginConfirm(title, action_label, cancel_label="Cancel")
replaces all five call sites, and makes "defaults to Cancel" a property of
the popup itself rather than something each new confirm has to remember.
Also drops two small redundancies spotted along the way: NearbyScreen's
_confirm.active = true, dead since begin() already sets it, and
RadioPresetPicker's deleting = false being set twice in a row (once inside
openConfirm(), once again by its only caller).

No behavior change; verified against the actual PopupMenu/menu-level state
machines in each of the five call sites before touching them.

examples/companion_radio/ui-new/AdminScreen.h

@@ -238,10 +238,7 @@ class AdminScreen : public UIScreen {
     } else if (f.set_prefix == nullptr) {                            // Action
       if (!strcmp(f.get_cmd, "start ota")) {                         // see _confirm's comment
         _pending_confirm_field = &f;
-        _confirm.begin("Start OTA update?", 2);
-        _confirm.addItem("Start");
-        _confirm.addItem("Cancel");
-        _confirm.setSelected(1);   // default highlight = Cancel
+        _confirm.beginConfirm("Start OTA update?", "Start");
         return;
       }
       strncpy(_cmd_text, f.get_cmd, sizeof(_cmd_text) - 1);